The invention relates to an imaging magnetic resonance apparatus with a magnet.
Imaging magnetic resonance apparatuses or nuclear magnetic resonance tomographs have a magnet for generating a constant magnetic field, for example an electromagnet coil. Electromagnetic waves necessary for exciting the atomic nuclei are generated by radio-frequency coils which emit pulsed waves. In the pauses, they receive a nuclear magnetic resonance signal emitted by the excited atomic nuclei. In order to be able to represent multidimensional body sections pictorially, it is necessary to determine the site of the origin of the emitted wave. For this purpose, in addition to the constant magnetic field a further magnetic field, which has a different magnitude at each site, is generated. This further magnetic field is generated by what is termed a gradient coil. The received nuclear magnetic resonance signals are fed to a computer in which the data are combined to form an image.
The requirements placed on the temporal stability of the spatially constant magnetic field are very high.
It is the object of the invention to specify an imaging magnetic resonance apparatus with a magnet in the case of which the constant magnetic field generated by the magnet is particularly stable over time.
This object is achieved according to the invention by virtue of the fact that the magnetic resonance apparatus has an isolation system for vibrational isolation of the magnet from a surrounding structure, in particular from a building floor, the isolation system having an actively controlled isolation module.
The invention proceeds in this case from the following consideration:
At a specific resonant frequency, the magnet can be excited to a natural vibration. The resonant frequency is usually below 100 Hz, for example at 16 Hz for a C-shaped permanent magnet with a structure in the shape of a tuning fork. A disturbing vibration, for example a building vibration, of just 65 to 75 dB below the gravitational acceleration (g) would suffice to excite this resonance so strongly that the investigations would be impaired. It is primarily the vertical component of the vibration that is disturbing in this case.
The invention further proceeds from the consideration that passive vibrational isolation alone is not sufficient for an imaging magnetic resonance apparatus. The reason for this is that passive vibrational isolation can be carried out only for a specific, unvarying frequency band by selecting the resonant frequency of the passive vibrational isolation system at an appropriate distance from the desired band. For example, a passive vibrational isolation system would have to be tuned to a very low resonant frequency of approximately  less than 4 Hz, in order to isolate sufficiently in the band of the natural vibration of 16 Hz of the magnet. However, this would have the effect that the passive vibrational isolation system would be caused to vibrate by the placement of the patient in the magnetic resonance apparatus, for example. These vibrations would likewise cause a disturbance of the magnetic field, in particular when iron is present in the surroundings.
It is not only the desired avoidance of vibrations generated by patients which would advocate a hard execution of a passive vibrational isolation system. A hard execution would also be advocated by the fact that the pulses in the hertz or subhertz bands generated by an expansion refrigeration machine for cooling superconducting magnets (what is termed a xe2x80x9ccold headxe2x80x9d) are to be led into the bottom in order not to disturb the magnet. In the expansion refrigeration machine, the displacer piston forms an accelerated mass of approximately 1.5 kg which moves to and fro approximately 10 cm roughly every 0.5 seconds. The aim is also to suppress the vibrations caused by the gradient coil in accordance with the loudspeaker principle. The hard execution, required for this purpose, of a passive vibrational isolation system would, however, run counter to the requirement for adequate vibrational isolation in the resonant range of the magnet.
An actively controlled isolation module is present according to the invention for the purpose of resolving this conflict. Said module can be operated flexibly in the case of vibrations of different frequency.
The isolation system is preferably integrated in a foot present for the purpose of supporting the magnet on the surrounding structure, or designed such that it can be fitted on the foot. Owing to such integration or fitting, the production outlay for the isolation system is low and a particularly efficient vibrational isolation is achieved.
According to a very particularly preferred refinement, the isolation system comprises a passive isolation module which preferably has a vibration-damping material, in particular rubber, and/or a spring element.
The combination of an actively controlled isolation module with a passive isolation module is particularly advantageous for suppressing vibration in the case of a magnetic resonance apparatus, since in this case disturbing vibrations occur in a very wide frequency band which already begins below 1 Hz and reaches into the kilohertz band. Vibrations in the kilohertz band which lead to disturbing noise are also generated by the gradient coil.
The audible noise generated by the gradient coil should not be coupled into the surrounding structure or the building. For this purpose, the passive isolation module is preferably designed to damp acoustic vibrations, in particular vibrations with a frequency of more than 50 Hz.
The actively controlled isolation module is particularly preferably designed to damp non-acoustic vibrations, in particular vibrations with a frequency of less than 50 Hz.
It is also possible to optimize the passive isolation module and the actively controlled isolation module for vibrational isolation in different frequency bands. For example, the passive isolation system is optimized to the effect that vibrations of the magnet in the audible frequency bands are not coupled into the surrounding structure or the building. The actively controlled isolation module is, for example, optimized to the effect that the magnet is isolated against the vibrations of the surrounding structure or the building in the low frequency range.
In particular, the actively controlled isolation module is set up to damp vibrations in the region of a resonant frequency of the magnet. In order to realize this damping, the actively controlled vibrational system can be optimized with particular efficiency if a passive isolation module is simultaneously present. A particularly high damping can be achieved in the case of the resonant frequency of the magnet with the aid of the actively controlled isolation module so as to achieve a particularly high level of temporal stability of the constant magnetic field generated by the magnet.
According to another particularly preferred embodiment, the actively controlled isolation module is integrated as final controlling element in a control loop which has a sensor. It is thereby advantageously possible reliably to suppress disturbing vibrations with a variant frequency and amplitude.
The sensor is preferably designed as an acceleration pickup and/or as a detector for measuring mechanical strain and/or as a detector for measuring a mechanical deflection.
The sensor is fitted with particular advantage on the magnet or in its vicinity. This means that the actively controlled isolation module can be adjusted in a targeted manner to the disturbing vibrations impinging on the magnet.
The actively controlled isolation module and the possibly present passive isolation module are preferably arranged one above another like a stack, and/or integrated in the foot.
According to another preferred embodiment, a further passive isolation module, for example a spring element, is present which acts in parallel with the actively controlled isolation module and/or with the passive isolation module. It is thereby advantageously possible for a portion of a static load, for example resulting from the mass of the magnet, to be absorbed or borne.
The sensor is preferably fitted on the final controlling element or in its vicinity. Particularly in the case of the said stack-type arrangement of the two isolation modules, there is the advantage that the influence of the passive isolation module on the control loop can be detected directly.
The actively controlled isolation module functioning as final controlling element preferably has a piezoelectric actuator. It is thereby possible to operate the actively controlled isolation module over a wide frequency band reaching the kilohertz.